Support Capital sharp S in German input methods

* lisp/leim/quail/latin-post.el ("german-postfix"):
* lisp/leim/quail/latin-pre.el ("german-prefix"): Add Capital
sharp S.  (Bug#30988)

* etc/NEWS: Mention the support of Capital sharp S.

etc/NEWS

@@ -121,6 +121,9 @@ regular expression was previously invalid, but is now accepted:
 
    x\{32768\}
 
+---
+** The German prefix and postfix input methods now support Capital sharp S.
+
 
 * Editing Changes in Emacs 27.1
 

lisp/leim/quail/latin-post.el

@@ -1103,6 +1103,7 @@ szz -> sz
  ("UE" ?Ü)
  ("ue" ?ü)
  ("sz" ?ß)
+ ("SZ" ?ẞ)
 
  ("AEE" ["AE"])
  ("aee" ["ae"])
@@ -1111,6 +1112,7 @@ szz -> sz
  ("UEE" ["UE"])
  ("uee" ["ue"])
  ("szz" ["sz"])
+ ("SZZ" ["SZ"])
  ("ge" ["ge"])
  ("eue" ["eue"])
  ("Eue" ["Eue"])

lisp/leim/quail/latin-pre.el

@@ -361,13 +361,14 @@ Key translation rules are:
  "german-prefix" "German" "DE>" t
  "German (Deutsch) input method with prefix modifiers
 Key translation rules are:
- \"A -> Ä ->   \"O -> Ö   \"U -> Ü   \"s -> ß
+ \"A -> Ä ->   \"O -> Ö   \"S -> ẞ   \"U -> Ü   \"s -> ß
 " nil t nil nil nil nil nil nil nil nil t)
 
 (quail-define-rules
  ("\"A" ?Ä)
  ("\"O" ?Ö)
  ("\"U" ?Ü)
+ ("\"S" ?ẞ)
  ("\"a" ?ä)
  ("\"o" ?ö)
  ("\"u" ?ü)